Process and apparatus for the production of metal oxides



Sept. 24, 1968 R..J. MAS ET Al.

PROCESS AND APPARATUS FOR THE PRODUCTION OF METAL OXIDES 4 4Sheets-Sheet 1 Filed July 16, 1963 INVENTORS ROBERT JEAN MAS ANDREL'OUIS MICHAUD A2ORNEY Sept. 24, 1968 Q R. J. MAS Ef AL 3,403,001

PROCESS AND APPARATUS FOR THE PRODUCTION OF METAL OXIDES Filed July 16,1963 4 Sheets-Sheet 2 INVENTORS ROBERT JEAN MAS A D E LOUIS MICHAUD AORNEY Sept. 24, 1968 R. J. MAS ET AL 3,403,001

PROCESS AND APPARATUS FOR THE PRODUCTION OF METAL OXIDES Filed July 16,1963 4 Sheets-Sheet 3 0 N N 8 co b010 0) OQ'NOIO A INVENTORS (D N N N ,CROBERT JEAN MAS ANDRE LOUIS MICHAUD AT ORNEY Sept. 24, 1968 R. J. MAS ETAL 3,403,001

PROCESS AND APPARATUS FOR THE PRODUCTION OF METAL OXIDES Filed July 16,1963 4 Sheets-Sheet i g n INVENTO S R0 RT JEAN MAS ANDR LOUIS MICHAUD vATT NEY United States Patent 3,403,001 PROCESS AND APPARATUS FOR THEPRO- DUCTION 0F METAL OXIDES Robert Jean Mas and Andr L. Michaud, Thann,France, assignors to Fabriques de Produits Chimiques de Thann et deMulhouse, Thann, France, a French body corporate Filed July 16, 1963,Ser. No. 295,489 Claims priority, application Great Britain, July 17,1962, 27,499/62; Feb. 8, 1963, 5,317/63 23 Claims. (Cl. 23202) Thisinvention relates to the production of finely-divided metal oxides bythe high temperature, vapour phase oxidation of metal chlorides. Theinvention is primarily concerned with the production of titanium dioxidefrom titanium tetrachloride, but the process and apparatus of theinvention may also be used for the oxidation of other volatile metalchlorides into the oxides of the corresponding metals. Examples ofchlorides which can be used include those of zirconium, aluminum, tin,chrominum, iron and silicon (which for the purpose of this invention isconsidered to be a metal).

Metal chlorides in the vapour phase may be burnt in oxygen at hightemperatures, e.g. above 1000 C., to give a suspension of metal oxideparticles in a chlorine-containing gas. The finely-divided oxideparticles thus produced make the oxide very suitable for many commercialapplications, e.g. as a pigment.

Many modifications of this general type of process are known. Forexample, the chloride vapour may be mixed, prior to combustion, with aninert diluent gas or with various substances said to improve the qualityof the oxide produced. Various burner arrangements have been used toensure adequate mixing of the metal chloride vapour and oxygen, and theperiod the oxide remains in the combustion zone has been closelycontrolled.

Three main categories of process may be specifically referred to:processes involving an auxiliary flame, i.e. a flame of a combustiblegas, usually carbon monoxide, into which the metal chloride and oxygenare introduced to induce combustion; processes in which no auxiliaryflame is used; and processes in which the combustion of the metalchloride takes place in a fluidized bed of the oxide produced by thecombustion. These types of process have certain disadvantages.

Thus, in processes in which an auxiliary flame is not used, it isnecessary to preheat the gaseous reactants, and this makes necessarylarge and complicated installations outside the reaction furnace.

Processes involving the use of a fluidized bed have the samedisadvantages and it is also necessary to keep the fluidized bed at ahigh, constant temperature.

In the processes in which an auxiliary flame is used, there is noproblem of heating the reactants, but the ditficulty of maintaining ahomogeneous and stable flame still exists. In known processes, thisdifliculty has not, so far, been completely overcome.

Thus, in one such process titanium tetrachloride, oxygen and nitrogenare introduced through a number of inlets arranged in a circle into theinterior of the auxiliary carbon monoxide flame which heats thereactants to combustion temperature. The carbon monoxide flame is formedof a relatively large number of individual flames which burn round theinlets through which the reactants are introduced. This arrangementgives a complex flame structure which is difficult to control closely,and the lack of control results in a non-uniform metal oxide product.

In another process a stream of hot burnt gases from an auxiliary flameis introduced into a cylindrical combustion chamber in a directiontangential to the walls of 3,403,001 Patented Sept. 24, 1968 "ice thesaid chamber and a mixture of metal chloride and oxygen is injected intothe spiralling stream of hot, burnt gases in a direction parallel withthe axis of the combustion chamber. In this process the flame producedby the combustion of the metal chloride (hereinafter referred to as thechloride flame) is separated from the auxiliary flame and control of thechloride combustion is, to some extent, facilitated, but the mixture ofmetal chloride and oxygen is not sufficiently rapidly mixed with hot,burnt gases for adequate control of the chloride flame to be possible,and the walls of the combustion chamber are corroded by the spirallinggases.

The process of the present invention comprises injecting intorectilinearly moving hot burnt gases passing at a high velocity from anauxiliary flame a mixture of metal chloride vapour and oxygen enteringthe hot gases in a direction which intersects the axis of the directionof motion of the said burnt gases. The mixture of chloride vapour andoxygen advantageously is injected in a plurality of separate streamsthereof spaced apart about the periphery of the stream of hot burntgases. The precise angle at which the mixture of metal chloride andoxygen is injected into the hot burnt gases may be selected as desiredbut is preferably from about 45 to about In the new flame processes, asin the process previously mentioned, the temperature of the mixture ofreactants (chloride and oxygen) is raised very suddenly from arelatively low temperature, above the dew point of the halide, to a verymuch higher temperature above the minimum temperature of reaction. Thus,the initial temperature of the mixture of chloride vapour and oxygen,with or without inert gas, may be between and C. while the burnt gasesinto which the mixture is injected may have a temperature of 1900 C.,and the consequent sudden heating of the reactants promotes homogeneityand constancy in the properties of the metal oxide obtained. However, inthe process of the present invention the burnt gases and the chlorideflame are kept away from the walls of the combustion chamber and thisprevents contamination of the metal oxide formed and corrosion of thewalls. Moreover, the mixture of metal chloride and oxygen is morerapidly mixed with the burnt gases from the auxiliary flame than in theprior processes and the chloride flame is consequently much bettercontrolled, and a more uniform product is obtained.

It has also been found that in the process of this in-. vention it isunnecessary to add to the feed streams, the activators and nucleiformers, such as silicon and aluminum chloride, which are generally usedin known processes to control the combustion of the metal chloride,especially titanium tetrachloride. In the process of the invention,nitrogen-containing products, which have favourable influences on thereaction, are formed in the auxiliary flame (because the gases fed tothe flame ordinarily contain nitrogen) and are subsequently mixed withthe reactants and improve the stability and homogeneity of the chlorideflame. It is thus unnecessary to add other activators or nuclei formers.The maximum number of nuclei is produced by the nuclei formers, whichinclude, for example, nitrogen oxides and ozone formed in the auixiliaryflame, when the initial temperature difference between the reactants andthe burnt gases is greatest.

The auxiliary flame is preferably, as in known processes, a carbonmonoxide flame. At its hottest part it may reach 2400 C. but the gasesleaving it are rather cooler, e.g. about 1900 C. The injection of themixture of metal chloride and oxygen causes a further lowering of thetemperature, e.g. to about 1400 C. The latter temperature may be reducedstill further by cooling the walls of the zone in which the chlorideflame burns, and it is often advantageous to effect such cooling.

The maximum temperature of the auxiliary flame can be reduced bysupplying excess oxygen over and above that required to convert all thecarbon monoxide to carbon dioxide, since this excess absorbs heat inbeing brought to flame temperature. Similarly, if oxygen is supplied tothe auxiliary flame in the form of air (as is normally the case) thenitrogen will absorb part of the heat of combustion of the carbonmonoxide.

The precise composition of the burnt gases from the auxiliary flamenaturally depends on the chemical composition of the combustible gasmixture supplied to the flame. At high temperatures, in the presence ofnitrogen and Water vapour, carbon monoxide burns to give a mixturecontaining carbon dioxide, water, hydrogen, carbon monoxide, oxygen,nitric oxide, and the H, O, and OH radicals. As the mixture cools ozoneand nitrogen peroxide may be produced. As already mentioned, some of theproducts of combustion of carbon monoxide in air, i.e. the products ofthe auxiliary flame, exert a beneficial effect on the combustion ofmetal chlorides by acting as nuclei formers for the formation of oxideparticles.

The invention is most advantageously applied to the production oftitanium dioxide, in a form suitable for use, inter alia, as a pigment,by the combustion of titanium tetrachloride. The titanium dioxideproduced has a very uniform grain size, the average maximum dimensionbeing less than 0.5a. The actual grain size may be varied somewhat byvarying the conditions of the process and thus the grain size of theproduct can be adapted for its intended use. As already explained, theoxide may be obtained without the use of activators or nuclei formersother than those produced by the auxiliary flame.

The invention includes within its scope a burner device for use in thenew process. This burner comprises a precombustion chamber, openings inthe said chamber for feeding oxygen and an inflammable gas thereto, atubular mixing chamber in open communication, and having a common axis,with the precombustion chamber, a plurality of conduits of smalldiameter opening into the said mixing chamber, the axis of each of whichintersects the axis of the mixing chamber, for feeding a mixture ofmetal chloride vapour and oxygen to the said mixing chamber, and meansfor cooling the walls of the said precombustion and mixing chambers. Themeans for cooling the walls of the precombustion and mixing chambers areprovided to lower the temperature of the chloride flame which, asmentioned above, is advantageous, especially in reducing corrosion ofthe mixing chamber.

The combustion of the chloride is, in general, not completed in themixing chamber but continues as the mixture of burnt gases, metalchloride and oxygen leaves the burner device and passes into thecontaing furnace. The metal forms a thick smoke in the furnace. Theburner may, of course, be provided with means for fixing it to thefurnace.

The volume of the precombustion chamber is preferably enough to ensurethat complete combustion of the carbon monoxide or other fuel in theauxiliary flame takes place in it but not so great that any appreciablecooling of the burnt gases takes place before they leave the chamber.The residence time of the gases in this chamber may be from some tenthsof seconds to as little as one or two hundredths of a second. Theprecise time will depend on the rapidity of combustion of theinflammable gas supplied, being, of course, least for gases of thehighest chemical reactivity with oxygen.

The actual size of the precombustion chamber and the mixing chamber willdepend on the consideration discussed in the last paragraph, thedisposition and number of the nozzles through which the chloride andoxygen are injected and the total output required of the burner device.

The invention includes within its scope installations for carrying outthe process of the invention comprising a furnace containing one or moreburner devices of the above type, means for discharging metaloxide-containing gases from the furnace, means for cooling andseparating the oxide, and means for recycling unspent gases. Such aconstruction ensures etficient use of the reactants and heat ofreaction, and thus leads to an improved yield of oxide and a reductionin overall size of the installation.

Suitable forms of burner device for use in the invention are shown, byway of example, in the accompanying drawings, wherein:

FIGURE 1 is a front section of a burner device with a cooled mixingchamber comprising pipes for the injection of the chloride atright-angles to the axis of the burnt FIGURE 2 is a front section of aburner device, the mixing chamber of which has been reduced in size, andin which the injection pipes are at an acute angle of about 45 to theburnt gas stream;

FIGURE 3 is a horizontal section along A-A of the burner device ofFIGURE 2 and shows the symmetrically disposed chloride injection pipes;and

FIGURE 4 is a front section of a burner device with a precombustionchamber and mixing chamber forming a Venturi tube.

Referring to FIGURE 1, A indicates a burner head with concentric pipes,B the refrigerating jacket of the body cooled by circulation of coldwater entering at inlet 11, and leaving at outlet :1 C the precombustionchamber in which the carbon monoxide or other auxiliary gas is burnt,and D is the chamber, formed as an axial extension of the combustionchamber and receiving hot burnt gases from the latter through aconstriction E increasing their velocity, for mixing the hot burnt gasesand the metal chloride and oxygen.

The oxygen is introduced into the burner head A by way of inlet g andenters the combustion chamber C through inlet g passing the fins I1 andh Carbon monoxide or other inflammable gas is introduced through inlet kand burns in the chamber C after having passed the fins k and k whichare provided to increase the turbulence of the gas.

Nitrogen can be introduced through inlet l and can be mixed with thecombustible gas, while the auxiliary flame and the combustion can beobserved through Window The mixture of, e.g., titanium tetrachloridewith an excess of oxygen enters the burner head A through inlet m Themixture reaches the mixing chamber by way of inlets m and 111 which leadinto conduits discharging through openings m and 111 as Well as throughother peripheral openings in symmetrical position, as shown in FIGURE 3.These openings may have various shapes, e.g. rectangular.

This burner device may be operated as follows: the auxiliary flame,usually of carbon monoxide, is set in operation and the rate of flow ofthe gas is regulated in an appropriate manner. The combustion of carbonmonoxide gives a very hot flame which may have a temperature higher than2000 C. (e.g. up to 2400 C.), and the burnt gases obtained are also veryhot. These gases are violently directed into the mixing chamber D, atthe entrance of which they meet the jets of chloride, which gives amixture which burns in the oxidation furnace proper, not shown in thefigure.

In burnt gases derived from carbon monoxide, carbon dioxidepredominates, but there are naturally other gaseous products which areformed at the various temperatures inside the flame. The formation ofthese other products leads to heat adsorption, thus causing atemperature drop in the burnt gases, which are cooled thereby usually toabout 1350 C. This temperature is suitable for obtaining rutile by thecombustion of titanium tetrachloride.

FIGURES 2, 3 and 4 also show improved burner devices in accordance withthe invention. These devices permit the introduction of chloride vapoursdiluted with oxygen into the hot burnt gases formed by the combustion ofcarbon monoxide. These burners are preferably of metal construction andmay, for example, be made of,

aluminum protected against corrosion and a too high temperature by acooling jacket through which flows a current of water or anotherappropriate fluid, such as an oil with a low vapour tension, e.g.Dowtherm (a eutectic mixture of diphenyl and diphenyl oxide). The oxygenstream entraining the chloride vapour passes through this iacket by ametal conduit, which is thermally insulated from the fluid in the mainjacket, thus avoiding excessive cooling of the vapours. The combustionof the auxiliary flame is practically complete when the gases reach thepoint at which the evaporized chloride is introduced. To avoid loss ofheat, the interior of the precombustion chamber is preferably lined withceramic material.

An especially advantageous burner arrangement according to the inventionis that shown in FIGURE 4, which represents a special burner deviceinside which the mixing chamber D forms, with the precombustion chamberC a Venturi tube having a constricted throat E; into which the halide isintroduced. The Venturi form makes it possible to obtain partial vacuumand a high velocity at the throat between the precombustion chamber andthe mixing chamber, and this facilitates mixing. The advantages of thisarrangement (when used for the combustion of titanium tetrachloride withan auxiliary flame of carbon monoxide) are:

(a) more rapid and efficient mixing of the gases;

(b) a substantial increase in the number of nuclei formers produced withbetter utilization thereof;

(c) increase in yield quantity of titanium dioxide per hour whilepermitting an economy in carbon monoxide; and

(d) the white pigments obtained have improved characteristics.

A flame produced, for example, by burning a mixture of oxygen and carbonmonoxide usually comprises two portions; a brilliant internal cone and aless brilliant external envelope. Although the combustion occurs mainlyat the internal surface of the internal cone, the combustion of thecarbon monoxide begins in the internal cone and the partially oxidisedproducts pass into the external envelope. A considerable quantity ofoxidised products or elements in the atomic or molecular state areformed at the external surface of the internal cone.

As the products pass into the progressively colder regions of theexternal envelope the combustion giving carbon dioxide continues untilthe reaction is complete. The heat liberated in the combustion dependson the degree of completion of the reaction and also on the localtemperature. The heat given off by the combustion exists in the form ofsensible heat in the gaseous combustion products and, as they areeliminated, the temperature of the combustion products is lowered.

The following examples illustrate the invention.

Example I An aluminum burner, such as that shown in FIGURE 1, isarranged in a cylindrical furnace.

The total height of-the apparatus is 150 cm. and its diameter is 45 cm.It is cooled by water at a rate of 2.5 cubic metres per hour and is fedwith 50 cubic metres per hour of oxygen, 80 cubic metres per hour ofcarbon monoxide, and 155 cubic metres per hour of a mixture of titaniumtetrachloride and oxygen, containing 33% by volume of the tetrachlorideand 67% by volume of oxygen.

It is possible to add to the reacting gases modifiers which have beenrecommended for modifying the structure of the product in a desirablemanner; for example, steam may be added to the oxidising gas or to thecarbon monoxide, or chlorides, such as aluminum chloride or siliconchloride, may be added to the stream of volatilised chloride.Nevertheless, as stated above, such additions are not essential and thenecessary nuclei formers appear to be formed in situ.

Example 11 In order to obtain titanium dioxide of pigmentary quality asrutile or anatase, an oxidising furnace of any desired type is used, butpreferably one which comprises a discharge device for the hot gasescontaining titanium dioxide, as described in French Patent No.1,307,280.

The burner shown in FIGURE 4 is arranged in this fur-.

nace.

Oxygen, at a rate of 45 cubic metres per hour, and carbon monoxidecontaining gas, at a rate of 70 cubic metres per hour, are introducedinto the burner. The combustible gas is made up of 75% by volume ofcarbon monoxide, 5% of carbon dioxide, and 20% of nitrogen. It isintroduced through a single pipe; that is to say, the pipe forintroducing nitrogen is not used. A mixture of titanium tetrachlorideand oxygen comprising 39% to 40% by volume of titanium tetrachloride isintroduced at a rate of 215 cubic metres per hour.

The reaction forming the titanium dioxide is entirely carried out insidethe oxidation furnace. 304 kg./hour of titanium dioxide are recovered,the satistical distribution of which from the granulometric point ofview is very interesting. The particle size distribution by weight andby number of different samples gives a very valuable indication of thepigmentary quality of different kinds of titanium dioxide. This isillustrated in FIGURES 5 and 6 of the accompanying drawings. FIGURE 5represents the statistical distribution, in percentages by weight as afunction of particle size in Angstrom units, of the particles in threesamples of pigment. The dotted curves represent two extreme types ofpigment having different pigmentary properties resulting from thedifferences in particle size distribution. The percentages by weight arecalculated from the formula:

statistical volume of all particles corresponding to a given sizestatistical volume of all the particles examined FIGURE 6 shows theparticle size distribution of the same three samples as in FIGURE 5 withthe difference that the percentage distribution by number of theparticles examined is plotted as a function of particle size. Each pointplotted on the curve represents the percentage of particles having adiameter within a given range of sizes. This range is obtained bygrouping together the mean values of the dimensions of the particles inorder to simplify the method of calculation and the representation ofthe results. The main diameter of the particles, 1922 angstroms (solidcurve in FIGURE 5), is obtained from the formula:

Example III A burner having precombustion and mixing chambers of thekind shown in FIGURE 1 is arranged in an oxidation furnace as describedin French Patent No. 1,307,280, from which titanium dioxide can beremoved continuously. The burner has the following principalcharacteristics. The precombustion chamber C is in the form of anelongated drum having a height of 530 mm. and a greatest diameter of mm.The mixing chamber D is cylindrical, with a diameter of 130 mm. and aheight of 390 mm. The injection orifices m and 111 have a diameter of 25mm. There are eight such orifices in all and they are disposedsymmetrically in the upper part of the mixing chamber. Their diameter isless than that of the orifice m through which the mixture of metallicchlorine and oxygen is introduced into the head of the burner. Thelatter also includes the separate orifices on entry k and g for carbonmonoxide and oxygen, respectively. The head of the burner is fed withthe reacting gases at rates which can be varied at will. The conditionsof operation are as follows:

The auxiliary flame is first produced as follows. Oxygen gas at a rateof 60 cubic metres per hour is introduced through orifice g and carbonmonoxide at a rate of 100 cubic metres per hour through orifice k andthe mixture is lit. A powerful, hot flame is thus produced. When thegases leaving the precombustion chamber C have heated the internal Wallsof the burner, a mixture of titanium tetrachloride and oxygen isintroduced through the orifice m at a rate of 55 cubic metres per hourof titanium tetrachloride and 72 cubic metres per hour of oxygen so thatthe mixed gases contain 44.5% by volume of titanium tetrachloride.

The chloride flame is thus produced and burns at the exit of the burner.If desired the walls of the burner can be cooled and the rate ofintroduction of the various gases varied. Titanium dioxide is producedcontinuously at a rate of 195 kg. per hour. At this rate of productionit is necessary to add 5 to 6 kg. of titanium dioxide to the furnace toform a store which can be recovered subsequently without interruption ofthe operation of the furnace. All the samples of product analysed are ofpigmentary quality and show excellent properties and particle sizedistribution.

Example IV The process of Example III is repeated with the same burnerarrangement, except that the injection orifices are four in number andhave a rectangular cross-section x 20 mm., permitting even distributionof the reaction gas. The auxiliary flame is obtained using cubic metresof oxygen per hour, introduced through orifice g and 90 cubic metres perhour of a gas containing by volume of carbon monoxide, introducedthrough orifice k The flame of metallic chloride is obtained byintroducing into the head of the burner 55 cubic metres per hour oftitanium tetrachloride and 82 cubic metres per hour of oxygen. Theproportion of titanium tetrachloride in the mixture is 40% by volume andthe mixture is introduced at an angle of to the burnt gases from theauxiliary flame. The chloride flame obtained in the furnace is long andstable. As in Example 111, titanium dioxide is produced very regularlyand the examination of samples shows it to be of fine pigment quality.

Example V Into an oxidation furnace permitting the recycling of all thecombustion gases to cool the titanium dioxide produced, as described inBritish specification No. 673,725,

a carbon monoxide burner having a mixing chamber of reduced size, asshown in FIGURE 2, is introduced. This burner includes the entryorifices necessary for the reaction gases. The diameters of the orificesfor admission of carbon monoxide and oxygen are respectively 65 mm. and40 mm. The tube for admitting the mixture of titanium tetrachloride andoxygen is mm. across. The total height of the head of the burner is 550mm., and the height of the precombustion chamber is 725 mm. There areeight orifices, each 18 mm. in diameter, for injecting the reactiongases at an angle of 45 to the gases from the auxiliary flame.

The burner is fed under the following conditions. The auxiliary flame isformed from 60 cubic metres per hour of oxygen and 100 cubic metres perhour of a gas containing 67% by volume of carbon monoxide. The chlorideflame is obtained by injecting a gaseous mixture of titaniumtetrachloride and oxygen through the aforesaid eight injection orifices.The mixture is formed from 67 cubic metres per hour of titaniumtetrachloride and 90 cubic metres per hour of oxygen and contains 42.5%by volume of titanium tetrachloride. The flame obtained in the furnaceis stable and homogeneous. It is sharply separated from the auxiliaryflame, as in other burners in accordance with the invention, althoughthe mixing chamber is much reduced in size. Titanium dioxide is producedat a rate of 236 kg. per hour, with 7 kg. of a recoverable residue, andis of pigmentary quality.

Example VI An oxidation apparatus is used which permits the recyclingthrough the burner of part of the oxidation gases, containing chloridebut from which titanium dioxide has been removed, as described in FrenchPatent No. 1,260,- 110. The burner of this apparatus is replaced by aburner of the type shown in FIGURE 4. The precombustion chamber C, has aheight of 530 mm. and a largest diameter of mm. The mixing chamber D hasa height of 330 mm. It tapers to form a Venturi towards theprecombustion chamber, the aperture of which has a diameter of 130 mm.The throat E of the Venturi contains eight holes, each having a diameterof 15 mm.

The reaction is carried out as in the preceding example. The auxiliaryflame is formed from 50 cubic metres per hour of oxygen and 80 cubicmetres per hour of a gas containing 70% by volume of carbon monoxide. Amixture of titanium tetrachloride and oxygen containing 25% by volume oftitanium tetrachloride is introduced at a rate of kg. per hour.

A large excess of oxygen does not vitiate the process of the inventionbecause it contributes to the formation of ozone, which acts as anactivator in the oxidation. As has already been mentioned, thetemperature of the carbon monoxide flame in the precombustion chamber isabout 2400 C. while the temperature of the gases leaving the combustionchamber is about 1900 C. In the mixing chamber the combustion gases mixwith the reaction gases (metal chloride plus oxygen) and the temperaturefalls to about 850 to 1350 C. The chloride flame burns in the furnacewith a long, stable flame and the temperature of the reaction is, onaverage, greater than 1150" C. These conditions of temperature andreaction enable titanium dioxide pigment to be obtained in the rutileform in the absence of any agent for promoting the formation of therutile allotrope.

The process of the invention can be applied not only to the oxidation oftitanium tetrachloride in the vapour phase but also to the oxidation inthe vapour phase of other metallic chlorides, such as those of aluminum,iron, zirconium, hafnium and niobium, all of which can he vaporized atbelow 500 C. at atmospheric pressure. Essentially the same technique andapparatus are used as that described above.

It can be shown that the titanium dioxide pigments which are obtained inaccordance with the invention are fine and regular, and it is thuspossible for them to be used, for example, in the pigmentation ofplastic materials.

We claim:

1. A process for the production of finely divided metal oxide, whichcomprises continuously forming hot burned gases in a combustion chamberby substantially completely burning a flammable fuel gas withoxygen-containing gas therein, continuously passing said hot burnedgases from said chamber in a stream thereof directed rectilinearlythrough a constriction substantially increasing its velocity, andcontinuously injecting into said stream, in a region thereof ofincreased velocity produced by said constriction, a gasous mixture ofmetal chloride vapor and oxygen containing at least the amount of oxygentheoretically required for complete oxidation of said vapor, therebycontinuously obtaining at the downstream side of said constriction areaction mixture composed of said burned gases and said gaseous mixture,in which said metal oxide is produced "by a steady flaming reaction.

2. A process according to claim 1, said gaseous mixture being injectedin a plurality of streams thereof separately entering said stream of hotIburned gases, each in a direction toward its axis, from locationsspaced apart about its periphery.

3. A process according to claim 1, said mixture being injected in aplurality of streams thereof separately entering said stream of hotburned gases, each at an angle of 45 to 90 to its axis, from locationsspaced apart about its periphery.

4. A process according to claim 1, said oxygen-com taining gascomprising air.

5. A process according to claim 1, said mixture of metal chloride vaporand oxygen being maintained, until injected into said hot burned gases,at a temperature insufficient to induce oxidation of said vapor.

6. A process according to claim 1, said gaseous mixture containingoxygen in excess of the amount theoretically required for completeoxidation of the metal chloride content.

7. A process for the production of a finely divided metal oxide, whichcomprises continuously forming in a combustion chamber burned gaseshaving a temperature of at least about 1900 C. by substantiallycompletely burning carbon monoxide with oxygen-containing gas in saidchamber, continuously passing said gases from said chamber in a streamthereof directed rectilinearly through a constriction substantiallyincreasing its velocity, and continuously injecting into said stream ofincreased velocity at the location of said constriction a gaseousmixture, directed toward the axis of said stream, of metal chloridevapor and oxygen for the oxidation of said vapor, thereby continuouslyobtaining at the downstream side of said constriction a homogeneousreaction mixture composed of said burned gases and said gaseous mixture,in which said metal oxide is produced by a steady flaming reaction.

8. A process according to claim 7, said constriction being the throat ofa Venturi tube and said gaseous mixture being injected in a plurality ofseparate streams thereof entering through openings in the throat of saidtube.

9. A process according to claim 7, said oxygen-containing gas comprisingair, and said gaseous mixture containing oxygen in excess of the amounttheoretically required for complete oxidation of the metal chloridecontent.

10. A process for the production of finely divided titanium dioxide,which comprises continuously forming hot burned gases at a temperatureof at least about 1900 C. in a combustion chamber by substantiallycompletely burning carbon monoxide with oxygen in the presence ofnitrogen in said chamber, continuously passing said burned gases fromsaid chamber in a stream thereof directed rectilinearly through aconstriction substantially increasing its velocity, and continuouslyinjecting into said stream, in a region thereof of increased velocityproduced by said constriction, a plurality of separate streams of agaseous mixture of titanium tetrachloride vapor and oxygen directed fromthe periphery of said stream toward its axis and containing an amount ofoxygen exceeding that theoretically required to oxidize said vapor totitanium dioxide, thereby continuously obtaining at the downstream sideof said constriction a reaction mixture composed of said burned gasesand said gaseous mixture, in which said titanium dioxide is producedcontinuously by a steady flaming reaction.

11. A process according to claim 10, said gaseous mixture being at atemperature between 120 and 150 C. and being injected into said streamof burned gases in a quantity forming with the latter a reaction mixturehaving a selected temperature in the range of 850 to 1400 C.

12. A process according to claim said gaseous mixture being at atemperature between 120 and 150 C. and :being injected into said streamof burned gases in a quantity forming with the latter a flaming reactionmixture having a temperature of at least about 1150 0, whereby saidtitanium dioxide is produced in the rutile form.

13. An apparatus for the production of finely divided metal oxide,comprising an elongated combustion chamber, means for continuouslyfeeding flammable gas and oxygen-containing gas into one end of saidchamber for combustion in said chamber to form hot substantiallycompletely burned gases continuously therein, an axially open tubularextension at the other end of said chamber for passing said burned gasescontinuously from said chamber to a reaction zone in a rectilinearlydirected stream, said extension comprising a constriction at said otherend for substantially increasing the velocity of said stream, and meansfor continuously injecting into said extension in a region thereof ofincreased velocity produced by said constriction, a gaseous mixture ofmetal chloride vapor and oxygen to oxidize said vapor, whereby ahomogeneous reaction mixture composed of said burned gases and saidgaseous mixture, for producing said metal oxide by a steady flamingreaction, is obtained in said extention at the downstream side of saidconstriction.

14. An apparatus according to claim 13, said injecting means including aplurality of conduits spaced apart about said tubular extension and eachopening thereinto in a direction toward the axis thereof, for deliveringa plurality of separate streams of said gaseous mixture into said streamof hot burned gases.

15. An apparatus according to claim 13, said tubular extension havingthe form of a Venturi tube.

16. An apparatus according to claim 13, said tubular extension havingthe form of a Venturi tube, and said injecting means including aplurality of conduits spaced apart about the throat of said tube andeach opening into said throat in a direction toward the axis thereof.

17. An apparatus according to claim 13, and means for cooling the wallsof said combustion chamber and said tubular extension.

18. An apparatus according to claim 13, and means including a jacketsurrounding said combustion chamber and said tubular extension forholding a cooling liquid in contact with their walls.

19. An apparatus according to claim 13, and means including a jacketsurrounding said combustion chamber and said tubular extension forholding a cooling liquid in contact with their walls, said injectingmeans including a head chamber surrounding a part of said feeding means,for receiving said gaseous mixture, and a plurality of heat-insulatedconduits extending from said head chamber through the liquid containingspace of said jacket to said tubular extension, each of said conduitsopening into said tubular extension in a direction toward the axisthereof.

20. An apparatus for the production of divided metal oxide, comprisingan elongated combustion chamber, means for continuously feeding fuel gasand oxygen-containing gas into one end of said chamber for combustion insaid chamber to form hot substantially completely burned gasescontinuously therein, an axially open tubular extension at the other endof said chamber for passing said burned gases continuously from saidchamber to a reaction zone in a rectilinearly directed stream, saidextension comprising a constriction at said other end for substantiallyincreasing the velocity of said stream, and means for continuouslyinjecting into said extension, in a region thereof of increased velocityproduced by said constriction, a gaseous mixture of metal chloride vaporand oxygen to oxidize said vapor, whereby a homogeneous reaction mixturecomposed of said burned gases and said gaseous mixture, for producingsaid metal oxide bya steady flaming reaction, is obtained in saidextension downstream of said constriction, and means including a jacketsurrounding said combustion chamber and said tubular extension forholding a cooling liquid in contact with their walls, said injectingmeans including a head chamber for receiving said gaseous mixture, saidhead chamber surrounding a part of said feeding means, and a pluralityof conduits spaced apart about said combustion chamber and 1 1 extendingfrom said head chamber through the liquid containing space of saidjacket to said tubular extension, each of said conduits opening intosaid extension in a direction toward the axis thereof.

21. An apparatus according to claim 20, said tubular extension and saidother end of said combustion chamber forming a Venturi tube the throatof which constitutes said constriction, said conduits opening radiallyinto said throat.

22. An apparatus according to claim 20, said combustion chambercomprising successively in the direction away from said burner means, anentrance portion of progressively increasing diameter, an intermediateportion of largest diameter and an exit portion progressively decreasingin diameter to said constriction.

23. An apparatus according to claim 20, said feeding means comprisingconcentric pipes for conducting separate streams of said fuel gas andsaid oxygen-containing gas coaxially into said combustion chamber.

References Cited UNITED STATES PATENTS Kinnaird 23--277 Weber et a123202 X Frey 23202 Schrader 23277 X Burden 23277 Burt et al. 23277 Allenet a1. 106300 Leistritz 23277 Ebner 231 Hellwig 23277 Nelson et a1.23202 Wagner 23277 Canada.

OSCAR R. VERTIZ, Primary Examiner.

E. STERN, Assistant Examiner.

1. A PROCESS FOR THE PRODUCTION OF FINELY DIVIDED METAL OXIDE, WHICHCOMPRISES CONTINUOUSLY FORMING HOT FURNED GASES IN A COMBINATION CHAMBERBY SUBSTANTIALLY COMPLETELY BURNING A FLAMMABLE FUEL GAS WITHOXYGEN-CONTAINING GAS THEREIN, CONTINUOUSLY PASSING SAID HOT BURNEDGASES FROM SAID CHAMBER IN A STREAM THEREOF DIRECTED RECTILINEARLYTHROUGH A CONSTRICTION SUBSTANTIALLY INCREASING IN VELOCITY, ANDCONTINUOUSLY INJECTING INTO SAID STREAM, IN A REGION THEREOF OFINCREASED VELOCITY PRODUCED BY SAID CONSTRICTION, A GASOUS MIXTURE OFMETAL CHLORIDE VAPOR AND OXYGEN CONTAINING AT LEAST THE AMOUNT OF OXYGENTHEORETICALLY REQUIRED FOR COMPLETE OXIDATION OF SAID VAPOR, THEREBYCONTINUOUSLY OBTAINING AT THE DOWNSTREAM SIDE OF SAID CONSTRICTION AREACTION MIXTURE COMPOSED OF SAID BURNED GASES AND SAID GASEOUS MIXTURE,IN WHICH SAID METAL OXIDE IS PRODUCED BY A STEADY FLAMING REACTION.